1. Field of the Invention
This invention relates to a print control method of electrophotography for rendering an image visible using coloring particles of toner, etc., of a printer, a facsimile, a copier, etc., and a recording apparatus using the method and in particular to a print control method in a print process consisting of charging, light exposure, developing, and transfer for forming a toner image on the surfaces of a photoconductor and record paper and an image formation apparatus using the method.
2. Description of the Related Art
As for the print control method of electrophotography, first a method in a related art will be discussed. An image formation apparatus using electrophotography includes a print process of rendering coloring particles visible on the surface of a record body as an image and a fixing process of fixing the coloring particle image rendered visible on the record body.
In the charging step, the full surface of the photoconductor is once charged and subsequently in the light exposure step, light is applied, thereby partially discharging. A potential contrast based on the charge area and the discharge area is formed on the surface of the photoconductor and is called an electrostatic latent image. In the developing step following the light exposure step, first the toner images of coloring particles are charged. As the toner charging method, a dual-component developing method using carrier beads or a mono component developing method of charging by friction with a toner member, etc., is available.
On the other hand, to render an electrostatic latent image visible, a method called bias developing is often used. In the bias developing, a bias voltage is applied to a developing roller for separating from the latent image potential formed on the surface of a photoconductor and the developer on the surface of the developing roller and moving to the surface of the photoconductor for forming an image. The above-mentioned charge potential or discharge potential may be used as the latent image potential. Generally, the method of using the charge potential as the latent image potential is called normal developing method and the method of using the discharge potential is called inverse developing method. The charge potential or discharge potential, whichever is unused as the latent image potential, is called background potential. The bias voltage of the developing roller is set midway between the charge potential and the discharge potential, and the difference between the bias voltage of the developing roller and the latent image potential is called developing potential difference. Likewise, the difference between the developing bias and the background potential is called background potential difference.
In the image formation apparatus of electrophotography, toner is jetted from the developing unit to the photoconductor surface in response to the latent image potential on the photoconductor for forming an image, and the image density changes with the toner amount for developing. It is generally known that the amount of toner jetted from the developing unit is proportional to the magnitude of the developing electric field, the electric field in the developing portion between the photoconductor and the developing unit. This developing electric field is noticeably observed in the edge part of a solid latent image and a line latent image. Thus, potential Vr2 called middle potential is provided between the developing bias and the latent image potential for reducing the toner deposition amounts on the edge part of the solid latent image and the line latent image. Formation of the electrostatic latent image and toner image on the photoconductor surface has been described.
Next, varying of the electrostatic latent image on the photoconductor surface with time will be discussed. When the photoconductor is degraded as the print amount grows, the charge area potential (charge potential) lowers and it becomes hard to charge. On the other hand, the discharge area potential (discharge potential) rises and it becomes hard to discharge. Lowering the discharge performance is remarkable if an intermediate potential area with incomplete discharge with an insufficient exposure light amount given is provided. This intermediate potential area mentioned here is often used for the purpose of thickness prevention, etc., in an image area where toner is too much developed with the strong peripheral effect of the electric field such as thin lines and dots. The described potential change acts in the direction of lowering the developing electric field to lessen the developing potential difference. On the other hand, in addition to the characteristic, the thickness of the photosensitive layer of the photoconductor decreases due to wear as the print amount grows. The decrease in the film thickness acts in the direction of increasing the developing electric field. Which of the two mutually contradictory tendencies is superior varies from one printing apparatus to another.
In any way, to keep the image quality constant over time, control needs to be performed for maintaining stable the potential of the latent image formed on the photoconductor and suppressing growing of the developing electric field because of decrease in the film thickness of the photoconductor. Generally, it is known that a potential sensor is used as means for detecting the potential on the photoconductor surface to perform such potential and electric field stabilizing control. For example, a method described in JP-A-11-15214 can be named as an art in a related art concerning such a surface potential control method of a photoconductor.
However, a potential sensor is placed between a light exposure unit and a developing device in the related art and thus it is necessary to provide an additional space for placing the potential sensor between the light exposure unit and the developing device. However, the distance between the light exposure point and the developing point is an area requiring strict design because of the light attenuation characteristic that the photoconductor has, and placing the potential sensor at such a position results in reception of every restriction. However, if the potential sensor is placed downstream in the photoconductor rotation direction from the developing device, it is impossible to measure the precise potential because of toner developing, namely, another problem arises.
In the described related art, the developing potential and the background potential on the photoconductor surface are changed so as to make the developing electric field constant and thus the image quality becomes stable in thin lines and dots with the range covered by the peripheral effect of the electric field as the main image areas, for example. However, in a wide solid area (solid image) where parallel and peripheral electric fields mix, etc., if stability of the image quality because of the peripheral effect of the electric field of the periphery is provided, a problem of lowering the density arises in the portion developed by the parallel electric field of the center.
It is an object of the invention to provide a print control method of an electrophotograph and an image formation apparatus of an electrophotograph wherein a potential sensor is placed in a post-transfer area where the packing density is comparatively sparse and at the place, the potential on the photoconductor drum surface at the developing point can be detected.
It is another object of the invention to provide a film thickness detection method of a photoconductor drum, fitted for an image formation apparatus wherein a potential sensor is placed in a post-transfer area.
It is another object of the invention to provide a print control method for keeping the image quality stable as time goes by if the photoconductor drum film thickness is changed in an image formation apparatus of an electrophotograph wherein a potential sensor is placed in a post-transfer area.
It is a further object of the invention to provide an image formation apparatus of an electrophotograph for printing a good image stably as time goes by wherein a potential sensor is placed in a post-transfer area.
One feature of the invention is characterized by a print control method of an electrophotograph in an image formation apparatus comprising at least a photoconductor, a charger, a light exposure unit, and a developing device for forming a background area and an image area on the photoconductor using the charger and the light exposure unit and detecting the potential of the image area after transfer and controlling the developing electric field, thereby printing an electrophotograph, wherein when the potential is detected, the toner covering percentage of the image area on the photoconductor is lowered.
Another feature of the invention is characterized by the fact that when the potential is detected, carrier fly suppression control is performed.
Another feature of the invention is characterized by a print control method in an image formation apparatus of an electrophotograph comprising at least a photoconductor, a charger, a light exposure unit, and a developing device for forming a background area and an image area on the photoconductor using the charger and the light exposure unit and detecting the potential of the image area after transfer, wherein a middle potential is set between a latent image potential and a developing bias, and wherein the film thickness of the photoconductor is detected and feedback control of the middle potential is performed so that the developing electric field becomes constant based on the detected film thickness.
According to the invention, a potential sensor is placed in a post-transfer area and at the position, the potential on the photoconductor drum surface at the developing point is detected. When the potential on the photoconductor drum surface is detected, the developing bias is avoided at the optimum timing and the potential is detected at the position after transfer. The correction potential amount grasped based on the in-machine humidity and the photoconductor drum film thickness previously measured is added to the detected potential and it is made possible to detect the potential on the photoconductor drum surface which is the same as the developing device position.
Feedback control is applied based on the corrected potential detection value, whereby the potential of the latent image formed on the photoconductor drum is kept stable as time goes by, the thickness of the photosensitive layer of the photoconductor drum is detected, the developing electric field is controlled based on the detected information, and change over time, caused by the thickness of the photosensitive layer is also eliminated.